Projection-type image display apparatus

ABSTRACT

Red, green and glue light beams from light source portions ( 301, 302, 303 ) are modulated respectively by light valve units ( 317, 319, 321 ) of a light valve portion ( 304 ) and enter a color combination optical system ( 305 ). The color combination optical system ( 305 ) includes three triangular prisms ( 325, 326, 327 ), each having a vertex angle of about 30 degrees, and is formed by joining the prisms together with dichroic mirror surfaces ( 328, 329 ) interposed therebetween. The respective light beams entering the planes of the prisms opposite to their vertex angles are combined in the color combination optical system ( 305 ) and emitted from an exit plane of the prism ( 327 ). The optical path lengths of the respective light beams between the incidence planes and the exit plane ( 332 ) are substantially equal to one another. Then, a projection lens ( 306 ) magnifies and projects the combined light beam onto a screen. This configuration can increase the quality of projection images and reduce the size and cost of the apparatus.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a three-plate projection-type imagedisplay apparatus including light valves (e.g., liquid crystal panels),one each for red, green and blue light beams, as a modulation means sothat display images of the respective light beams are combined in theapparatus and projected to form a magnified image on a screen.

2. Background Art

The projector market, especially for projection-type image displayapparatuses using a transmission-type liquid crystal panel, now isgrowing rapidly. The trends of products can be divided into two majorcategories: higher brightness and smaller size. In particular, thediagonal size of an effective aperture of a liquid crystal panel isreduced from 1.3 inches, which has been a mainstream diagonal size, to0.9 inches at present and is expected to be reduced further in thefuture. While reducing the effective aperture size, thetransmission-type liquid crystal panel has a very small black matrix(BM) and a numerical aperture high enough to be comparable with that ofa conventional liquid crystal panel that is one size larger than theabove liquid crystal panel. With the implementation of such a small-sizehigh-density liquid crystal panel, a color combination portion forcombining display images on the liquid crystal panels also needs toprovide higher accuracy.

Next, the configuration of a conventional projection-type image displayapparatus using liquid crystal panels will be described. Three-plateprojection-type image display apparatuses including liquid crystalpanels, one each for red, green and blue light beams, can be classifiedroughly into two categories according to their characteristics in colorcombination: a cross-prism system and a mirror-sequential system. FIGS.7 and 8 schematically show the basic configurations of conventionalprojection-type image display apparatuses employing the cross-prismsystem and the mirror-sequential system, respectively. The following isan explanation for each of the configurations.

As shown in FIG. 7, a cross-prism projection-type image displayapparatus 100 includes a light source portion 101, a color separationoptical system 102, a relay optical system 103, a light valve portion104, a color combination optical system 105, and a projection opticalsystem (a projection lens) 106.

The light source portion 101 includes a light source 107 and a reflector108. The light source 107 forms an arc by discharge between electrodesto generate a randomly polarized light beam. The reflector 108 reflectsthe light beam from the light source 107 in one direction along its axisof rotational symmetry.

A light beam from the light source portion 101 enters a blue-reflectiondichroic mirror 109 of the color separation optical system 102, where ablue light beam of the incident light is reflected. Then, the blue lightbeam is reflected from a total reflection mirror 110 and passes througha condenser lens 111 into a blue light valve unit 112. Green and redlight beams are transmitted by the blue-reflection dichroic mirror 109and enter a green-reflection dichroic mirror 113, where the green lightbeam is reflected and passes through a condenser lens 114 into a greenlight valve unit 115. The red light beam is transmitted by thegreen-reflection dichroic mirror 113 and enters the relay optical system103. Then, the red light beam passes through an entrance lens 116, atotal reflection mirror 117, an intermediate lens 118, a totalreflection mirror 119, and a condenser lens 120 into a red light valveunit 121.

The light valve portion 104 includes the blue, green and red light valveunits 112, 115 and 121, which are arranged in accordance with therespective light beams. Each of the light valve units 112, 115 and 121includes an entrance polarizing plate 122, a liquid crystal panel 123,and an exit polarizing plate 124, as shown in FIG. 2. The entrancepolarizing plate 122 is rectangular in shape and designed, e.g., totransmit light polarized in the short side direction and to absorb lightpolarized in the direction perpendicular thereto. The light beam passingthrough the entrance polarizing plate 122 enters the liquid crystalpanel 123. The liquid crystal panel 123 has many pixels arranged in theform of an array and can change the polarization direction of theincident light at each pixel aperture with an external signal. In thisexample, when the pixels are not driven, the liquid crystal panel 123transmits the incident light while rotating its polarization directionby 90 degrees; when the pixels are driven, the liquid crystal panel 123transmits the incident light without changing its polarizationdirection. The exit polarizing plate 124 has polarizationcharacteristics in the direction perpendicular to the entrancepolarizing plate 122. In other words, the exit polarizing plate 124 hasa transmission axis in the long side direction of its rectangularoutline and transmits light polarized in this direction. Therefore, thelight beam that has entered the undriven pixel of the liquid crystalpanel 123 and been transmitted with its polarization direction rotatedby 90 degrees can pass through the exit polarizing plate 124 because itis polarized in the direction parallel to the transmission axis. On theother hand, the light beam that has entered the driven pixel of theliquid crystal panel 123 and been transmitted with its polarizationdirection unchanged is absorbed by the exit polarizing plate 124 becauseit is polarized in the direction perpendicular to the transmission axis.

The light beams thus transmitted through the light valve portion 104enter the color combination optical system 105. The color combinationoptical system 105 is a color combination prism formed by joining fourtriangular prisms so that a blue-reflection dichroic mirror surface 125and a red-reflection dichroic mirror surface 126 cross at right angles.The blue and red light beams incident on the color combination opticalsystem 105 are reflected from the blue-reflection dichroic mirrorsurface 125 and the red-reflection dichroic mirror surface 126,respectively, and then enter the projection lens 106, which acts as aprojection optical system. The green light beam passes through the blue-and red-reflection dichroic mirror surfaces 125, 126 and enters theprojection lens 106.

The projection lens 106 magnifies and projects the incident light onto ascreen (not shown). In this manner, images of three light beams, each ofwhich is formed in the light valve portion 104, are combined anddisplayed as a color image.

As shown in FIG. 8, a mirror-sequential projection-type image displayapparatus includes a light source portion 201, a color separationoptical system 202, a light valve portion 203, a color combinationoptical system 204, and a projection optical system (a projection lens)205.

The light source portion 201 includes a light source 206 and a reflector207. The light source 206 forms an arc by discharge between electrodesto generate a randomly polarized light beam. The reflector 207 reflectsthe light beam from the light source 206 in one direction along its axisof rotational symmetry.

A light beam from the light source portion 201 enters a blue-reflectiondichroic mirror 208 of the color separation optical system 202, where ablue light beam of the incident light is reflected. Then, the blue lightbeam is reflected from a total reflection mirror 209 and passes througha condenser lens 210 into a blue light valve unit 211. Green and redlight beams are transmitted by the blue-reflection dichroic mirror 208and enter a green-reflection dichroic mirror 212, where the green lightbeam is reflected and passes through a condenser lens 213 into a greenlight valve unit 214. The red light beam is transmitted by thegreen-reflection dichroic mirror 212 and passes through a condenser lens215 into a red light valve unit 216.

The light valve portion 203 includes the blue, green and red light valveunits 211, 214 and 216, which are arranged in accordance with therespective light beams. Each of the light valve units 211, 214 and 216includes an entrance polarizing plate 217, a liquid crystal panel 218,and an exit polarizing plate 219, as shown in FIG. 2. The entrancepolarizing plate 217 is rectangular in shape and designed, e.g., totransmit light polarized in the short side direction and to absorb lightpolarized in the direction perpendicular thereto. The light beam throughthe entrance polarizing plate 217 enters the liquid crystal panel 218.The liquid crystal panel 218 has many pixels arranged in the form of anarray and can change the polarization direction of the incident light ateach pixel aperture with an external signal. In this example, when thepixels are not driven, the liquid crystal panel 218 transmits theincident light while rotating its polarization direction by 90 degrees;when the pixels are driven, the liquid crystal panel 218 transmits theincident light without changing its polarization direction. The exitpolarizing plate 219 has polarization characteristics in the directionperpendicular to the entrance polarizing plate 217. In other words, theexit polarizing plate 219 has a transmission axis in the long sidedirection of its rectangular outline and transmits light polarized inthis direction. Therefore, the light beam that has entered the undrivenpixel of the liquid crystal panel 218 and been transmitted with itspolarization direction rotated by 90 degrees can pass through the exitpolarizing plate 219 because it is polarized in the direction parallelto the transmission axis. On the other hand, the light beam that hasentered the driven pixel of the liquid crystal panel 218 and beentransmitted with its polarization direction unchanged is absorbed by theexit polarizing plate 219 because it is polarized in the directionperpendicular to the transmission axis.

The light beams thus transmitted through the light valve portion 203enter the color combination optical system 204. The color combinationoptical system 204 includes a green-reflection dichroic mirror 220, ared-reflection dichroic mirror 221, and a total reflection mirror 222.The blue light beam emitted from the blue light valve unit 211 passesthrough the green-reflection dichroic mirror 220 and the red-reflectiondichroic mirror 221 in sequence and enters the projection lens 205,which acts as a projection optical system. The green light beam emittedfrom the green light valve unit 214 is reflected from thegreen-reflection dichroic mirror 220, passes through the red-reflectiondichroic mirror 221, and enters the projection lens 205. The red lightbeam emitted from the red light valve unit 216 is reflected from thetotal reflection mirror 222 and the red-reflection dichroic mirror 221in sequence and enters the projection lens 205.

The projection lens 205 magnifies and projects the incident light onto ascreen (not shown). In this manner, images of three light beams, each ofwhich is formed in the light valve portion 203, are combined anddisplayed as a color image.

The above two projection-type image display apparatuses have typicalconfigurations currently used for presentation, and theircharacteristics will be described below.

The projection-type image display apparatus using the cross-prism systemfor color combination (FIG. 7) has the advantages that (1) the focallength and size of the projection lens can be reduced because theprojection distance between each of the liquid crystal panels and theprojection lens is made shorter, and (2) the accuracy can be ensuredeasily under vibration and shock because the color combination opticalsystem has a small size and the reflection planes are formed of prisms.However, there are problems as follows: (1) when the four prisms of thecolor combination optical system 105 are not joined together withsufficient accuracy, a vertical line appears on the center of aprojection image due to the interface between the prisms; (2) each ofthe reflection planes 125, 126 of the color combination optical system105 is formed by arranging two prisms so that a dichroic mirror surfaceof one prism is flush with that of the other prism, and thus colorirregularity is caused if the two dichroic mirror surfaces of eachreflection plane do not have the same spectral characteristic; (3)defocus of a projection image, such as a double image, occurs unless thedichroic mirror surfaces of two prisms that form each of the reflectionplanes 125, 126 are flush with each other without any distortion anddeviation; and (4) the relay optical system 103 is needed in addition tothe color separation optical system 102, which increases the apparatussize and also leads to color irregularity when the light source orillumination optical system has non-uniform brightness because the lightsource image of a light beam that passes through the relay opticalsystem is reversed with respect to the light source images of two otherlight beams that do not pass though the relay optical system.Considering the improvement in accuracy of the color combination opticalsystem that accompanies the use of such a high definition liquid crystalpanel described above, the problems (1) and (3) particularly have to besolved. Therefore, it is necessary to enhance machining accuracy of thecolor combination optical system further.

The projection-type image display apparatus using the mirror-sequentialsystem for color combination (FIG. 8) has the advantages that (1) theapparatus is relatively inexpensive and adapted easily to a large liquidcrystal panel, (2) the apparatus can reduce the weight, and (3) in theabsence of a relay optical system, the apparatus size can be relativelysmall and nonuniformity in brightness of the light source portion haslittle effect on projection images. However, there are problems asfollows: (1) since a light beam passes through obliquely placed parallelplanes, an astigmatic difference is caused, shifting the position of afocus on a vertical line from that on a horizontal line, which resultsin a blurred projection image; (2) it is difficult to provide flatnessof the dichroic mirror surface formed on a thin glass sheet, whichresults in a blurred projection image; and (3) an increase in size ofthe color combination optical system 204 makes it difficult to achievemechanical strength, to resist an external force such as vibration, andto maintain convergence accuracy. In particular, (1) and (2) are seriousproblems in promoting small size and high definition of a liquid crystalpanel. Thus, the cross-prism system so far has gained mainstream use,though there remain the above problems to be solved.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a projection-typeimage display apparatus including a novel optical system that canovercome the above problems of various types of conventional opticalsystems, arising when the apparatus uses small-size high-definitionliquid crystal panels.

To achieve the object, the present invention has the followingconfigurations.

A first projection-type image display apparatus of the present inventionincludes the following: three light source portions for emitting red,green and blue light beams, respectively; a light valve portion formodulating each of the light beams from the light source portions; acolor combination optical system for combining the light beams modulatedby the light valve portion; and a projection lens for magnifying andprojecting the combined light beam. The color combination optical systemincludes three triangular prisms, each of which has a vertex angle ofabout 30 degrees (preferably 27 to 33 degrees, more preferably 29 to 31degrees, and most preferably 30 degrees), and is formed by joining thethree prisms together so that the side faces of each prism that form thevertex angle are brought into contact to make the vertex angle of oneprism next to that of the other prism. Each of the joining planesbetween the prisms is provided with a dichroic mirror surface acting asa color selection means. The side face of each prism opposite to thevertex angle is used as an incidence plane for each of the light beams.The side face of the prism arranged at one end of the three joinedprisms is used as an exit plane for the combined light beam. The opticalpath lengths of the respective light beams between the incidence planesand the exit plane are substantially equal to one another.

Instead of the three light source portions, the present invention canemploy a light source portion for emitting a white light beam. A secondprojection-type image display apparatus of the present inventionincludes the following: a light source portion for emitting a whitelight beam; a color separation optical system for separating the whitelight beam from the light source portion into red, green and blue lightbeams; a light valve portion for modulating each of the light beams fromthe color separation optical system; a color combination optical systemfor combining the light beams modulated by the light valve portion; anda projection lens for magnifying and projecting the combined light beam.The color combination optical system includes three triangular prisms,each of which has a vertex angle of about 30 degrees (preferably 27 to33 degrees, more preferably 29 to 31 degrees, and most preferably 30degrees), and is formed by joining the three prisms together so that theside faces of each prism that form the vertex angle are brought intocontact to make the vertex angle of one prism next to that of the otherprism. Each of the joining planes between the prisms is provided with adichroic mirror surface acting as a color selection means. The side faceof each prism opposite to the vertex angle is used as an incidence planefor each of the light beams. The side face of the prism arranged at oneend of the three joined prisms is used as an exit plane for the combinedlight beam. The optical path lengths of the respective light beamsbetween the incidence planes and the exit plane are substantially equalto one another.

According to the first and second configurations, the color combinationoptical system is formed as a prism block in which three prisms arejoined together. This makes it possible to increase mechanical strength,to maintain durability, and to ensure accuracy even if an external forcesuch as vibration is applied after convergence has been adjusted, thusproviding an optical system with high reliability.

Moreover, all the reflection planes of the color combination opticalsystem are the side faces of a single prism. Therefore, thisconfiguration can overcome such problems of the cross-prism system thata vertical line (shadow) appears on the center of a screen due to theinterface between the prisms, color irregularity is caused by thedifference in spectral characteristic between two prism surfaces thatform one reflection plane, and defocus such as a double image occursbecause the two prism surfaces are not flush with each other.

Unlike the cross-prism system, there is no need to align a surface ofone prism with that of the other prism so as to form the same plane forjoining. Thus, the cost can be reduced.

Unlike the mirror-sequential system, a chief ray does not pass thoroughobliquely placed parallel planes. Therefore, images are not blurred.Since the dichroic mirror surface is formed on the side face of a prism,plane accuracy can be achieved easily and images are not blurred.

The distance between the light valve portion and the projection lens(i.e., a back focal length of the projection lens) can be minimized,thus reducing the size and cost of the projection lens.

The use of glass prisms allows the optical paths in the colorcombination optical system to be filled with glass, so that the opticalpath length can be made relatively short (specifically, though it may belonger than the optical path length in the cross-prism system, it issignificantly shorter than that in the mirror-sequential system). Thus,the size of the apparatus can be reduced.

In the first and second apparatuses, it is preferable that the threeprisms of the color combination optical system are first, second andthird prisms that are joined in this order; a first dichroic mirrorsurface is provided at the joining plane between the first prism and thesecond prism, and a second dichroic mirror surface is provided at thejoining plane between the second prism and the third prism; the exitplane is the side face of the third prism other than the joining planeand the incidence plane; a light beam entering the incidence plane ofthe first prism passes through the first prism, the first dichroicmirror surface, the second prism, the second dichroic mirror surface,and the third prism in sequence and exits from the exit plane; a lightbeam entering the incidence plane of the second prism passes through thesecond prism, is reflected from the first dichroic mirror surface topass through the second prism again, passes through the second dichroicmirror surface and the third prism, and exits from the exit plane; and alight beam entering the incidence plane of the third prism passesthrough the third prism, is reflected from the side face including theexit plane to pass through the third prism again, is reflected from thesecond dichroic mirror surface to pass through the third prism yetagain, and exits from the exit plane.

This preferred configuration can facilitate the combination of the threelight beams and also make their optical path lengths equal.

In the preferred configuration, it is preferable that both the lightbeams entering the second and third prisms are s-polarized light withrespect to the first and second dichroic mirror surfaces. Moreover, itis preferable that the light beam entering the first prism isp-polarized light with respect to the first and second dichroic mirrorsurfaces.

This preferred configuration can increase the utilization efficiency oflight from the light source.

It is preferable that the light beam entering the first prism is a greenlight beam.

This preferred configuration can increase the utilization efficiency oflight from the light source.

In the first and second apparatuses, it is preferable that the threeprisms of the color combination optical system are of the same shape.

This preferred configuration can reduce the cost of the colorcombination optical system.

In the second apparatus, it is preferable that the light valve portionincludes three light valves, one each for the respective light beams;the color separation optical system includes at least two dichroicmirrors and three reflection mirrors, the dichroic mirrors separatingthe white light beam from the light source portion into the red, greenand blue light beams, and the reflection mirrors being arranged inaccordance with the three light valves so as to guide the separatedlight beams to the corresponding light valves; and the optical pathlengths of the three light beams between the light source portion andthe light valves are substantially equal to one another.

Specifically, it is preferable that the three prisms of the colorcombination optical system are first, second and third prisms that arejoined in this order; the exit plane is the side face of the third prismother than the plane joined to the second prism and the incidence plane;the light valve portion includes first, second and third light valves,one each for the respective light beams; the first, second and thirdlight valves are arranged opposite to the incidence planes of the first,second and third prisms, respectively; the color separation opticalsystem includes at least first and second dichroic mirrors and first,second and third reflection mirrors; the first dichroic mirror separatesa third light beam from the white light beam emitted by the light sourceportion, and then the second dichroic mirror separates first and secondlight beams; the first light beam is reflected from the first reflectionmirror, passes through the first light valve, and enters the incidenceplane of the first prism; the second light beam is reflected from thesecond reflection mirror, passes through the second light valve, andenters the incidence plane of the second prism; the third light beam isreflected from the third reflection mirror, passes through the thirdlight valve, and enters the incidence plane of the third prism; and theoptical path lengths of the three light beams between the light sourceportion and the light valves are substantially equal to one another.

According to this preferred configuration, the color separation opticalsystem does not require a relay optical system. Therefore, the size andcost of the apparatus can be reduced. Moreover, the optical path lengthsof the three light beams between the light source portion and therespective light valves are substantially equal to one another. Thus,this configuration does not cause the problem of color irregularityresulting from a reverse of the light source image due to a differencein the optical path lengths, which arises along with the use of a relayoptical system. Consequently, high image quality can be achieved.

In the above preferred configuration, the optical axis that goes throughthe first dichroic mirror and the first reflection mirror may besubstantially orthogonal to the optical axis that goes through the firstreflection mirror and the exit plane, and thus a chief ray of the whitelight beam can enter the first dichroic mirror at the angle of incidencesmaller than 45 degrees.

Alternately, the optical axis that goes through the first dichroicmirror and the third reflection mirror may be substantially parallel tothe optical axis that goes through the first reflection mirror and theexit plane, and thus a chief ray of the white light beam can enter thefirst dichroic mirror at the angle of incidence larger than 45 degrees.

In the first and second apparatuses, it is preferable that light emittedfrom the light source portion is polarized light having a uniformpolarization direction.

This preferred configuration can improve the utilization efficiency oflight from the light source portion. When a liquid crystal light valveis used in the light valve portion, this configuration can reduceoptical absorption by an entrance polarizing plate.

In the first and second apparatuses, it is preferable that the lightvalve portion includes three light valve units, one each for therespective light beams, and each of the light valve units includes atleast an entrance polarizing plate as a polarizer, a transmission-typeliquid crystal panel, and an exit polarizing plate as an analyzer.

This preferred configuration can form images with a simple structure.

In the first and second apparatuses, it is preferable that the base ofeach of the triangular prisms is a right triangle.

This preferred configuration can make the optical path lengths of therespective light beams in the color combination optical system equal.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing the configuration of aprojection-type image display apparatus according to Embodiment 1 of thepresent invention.

FIG. 2 is a perspective view showing the schematic configuration of alight valve unit used in a projection-type image display apparatus ofthe present invention.

FIG. 3 is a schematic view showing the configuration of aprojection-type image display apparatus according to Embodiment 2 of thepresent invention.

FIG. 4 is a schematic view showing another configuration of aprojection-type image display apparatus according to Embodiment 2 of thepresent invention.

FIG. 5 is a schematic view showing the configuration of aprojection-type image display apparatus according to Embodiment 3 of thepresent invention.

FIG. 6 shows the configuration of a polarization direction convertingoptical system used in a projection-type image display apparatusaccording to Embodiment 3 of the present invention.

FIG. 7 is a schematic view showing the configuration of a conventionalcross-prism projection-type image display apparatus.

FIG. 8 is a schematic view showing the configuration of a conventionalmirror-sequential projection-type image display apparatus.

DETAILED DESCRIPTION OF THE INVENTION Embodiment 1

FIG. 1 is a schematic view showing the configuration of aprojection-type image display apparatus according to Embodiment 1 of thepresent invention.

A projection-type image display apparatus 300 of this embodimentincludes a red light source portion 301, a blue light source portion302, a green light source portion 303, a light valve portion 304, acolor combination optical system 305, and a projection optical system (aprojection lens) 306.

The red light source portion 301 includes a light source 307, areflector 308, and a red-transmission dichroic filter 309. The lightsource 307 forms an arc by discharge between electrodes to generate arandomly polarized white light beam. The reflector 308 reflects thelight beam from the light source 307 in one direction along its axis ofrotational symmetry. The red-transmission dichroic filter 309 is locatedahead of the opening of the reflector 308.

The blue light source portion 302 includes a light source 310, areflector 311, and a blue-transmission dichroic filter 312. The lightsource 310 forms an arc by discharge between electrodes to generate arandomly polarized white light beam. The reflector 311 reflects thelight beam from the light source 310 in one direction along its axis ofrotational symmetry. The blue-transmission dichroic filter 312 islocated ahead of the opening of the reflector 311.

The green light source portion 303 includes a light source 313, areflector 314, and a green-transmission dichroic filter 315. The lightsource 313 forms an arc by discharge between electrodes to generate arandomly polarized white light beam. The reflector 314 reflects thelight beam from the light source 313 in one direction along its axis ofrotational symmetry. The green-transmission dichroic filter 315 islocated ahead of the opening of the reflector 314.

A red light beam from the red light source portion 301 passes through acondenser lens 316 into a red light valve unit 317. A blue light beamfrom the blue light source portion 302 passes through a condenser lens318 into a blue light valve unit 319. A green light beam from the greenlight source portion 303 passes through a condenser lens 320 into agreen light valve unit 321.

The light valve portion 304 includes the red, blue and green light valveunits 317, 319 and 321, which are arranged in accordance with therespective light beams. Each of the light valve units 317, 319 and 321includes an entrance polarizing plate 322, a liquid crystal panel 323,and an exit polarizing plate 324, as shown in FIG. 2. The entrancepolarizing plate 322 is rectangular in shape and designed, e.g., totransmit light polarized in the short side direction and to absorb lightpolarized in the direction perpendicular thereto. The light beam passingthrough the entrance polarizing plate 322 enters the liquid crystalpanel 323. The liquid crystal panel 323 has many pixels arranged in theform of an array and can change the polarization direction of theincident light at each pixel aperture with an external signal. In thisembodiment, when the pixels are not driven, the liquid crystal panel 323transmits the incident light while rotating its polarization directionby 90 degrees; when the pixels are driven, the liquid crystal panel 323transmits the incident light without changing its polarizationdirection. The exit polarizing plate 324 has polarizationcharacteristics in the direction perpendicular to the entrancepolarizing plate 322. In other words, the exit polarizing plate 324 hasa transmission axis in the long side direction of its rectangularoutline and transmits light polarized in this direction. Therefore, thelight beam that has entered the undriven pixel of the liquid crystalpanel 323 and been transmitted with its polarization direction rotatedby 90 degrees can pass through the exit polarizing plate 324 because itis polarized in the direction parallel to the transmission axis. On theother hand, the light beam that has entered the driven pixel of theliquid crystal panel 323 and been transmitted with its polarizationdirection unchanged is absorbed by the exit polarizing plate 324 becauseit is polarized in the direction perpendicular to the transmission axis.

The light beams thus transmitted through the light valve portion 304enter the color combination optical system 305.

The color combination optical system 305 is formed by joining threetriangular prisms (i.e., a first prism 325, a second prism 326 and athird prism 327) together. The three prisms are of the same shape, andthe base of each prism is a right triangle having an interior angle of30 degrees (hereinafter, referred to as a vertex angle). As shown inFIG. 1, the three prisms 325, 326 and 327 are joined in this order sothat their vertex angles are next to each other. The side faces 325 a,326 a and 327 a opposite to the vertex angles of the first, second, andthird prisms 325, 326 and 327 are opposite to the light valve units 321,319 and 317, respectively. A blue-reflection dichroic mirror coatedsurface (a first dichroic mirror surface) 328 is formed at the joiningplane between the first prism 325 and the second prism 326. Similarly, ared-reflection dichroic mirror coated surface (a second dichroic mirrorsurface) 329 is formed at the joining plane between the second prism 326and the third prism 327. The incidence plane 325 a for the green lightbeam (i.e., the side face of the first prism 325 opposite to the greenlight valve unit 321) is provided with a λ/2 phase-difference plate 331.

The green light beam emitted from the green light valve unit 321 passesthrough the λ/2 phase-difference plate 331, where its polarizationdirection is twisted by 90 degrees. The green light beam thus twisted isp-polarized light with respect to the blue- and red-reflection dichroicmirror coated surfaces 328, 329. The green light beam enters the sideface 325 a (a first incidence plane) of the first prism 325, passesthrough the first prism 325, the blue-reflection dichroic mirror coatedsurface 328, the second prism 326, the red-reflection dichroic mirrorcoated surface 329, the third prism 327, and the side face of the thirdprism (an exit plane 332) in sequence, and enters the projection lens306, which acts as a projection optical system.

The blue light beam emitted from the blue light valve unit 319 iss-polarized light with respect to the blue- and red-reflection dichroicmirror coated surfaces 328, 329. The blue light beam enters the sideface 326 a (a second incidence plane) of the second prism 326, passesthrough the second prism 326, and is reflected from the blue-reflectiondichroic mirror coated surface 328 to pass through the second prism 326again. Then, it passes through the red-reflection dichroic mirror coatedsurface 329, the third prism 327, and the exit plane 332 and enters theprojection lens 306.

The red light beam emitted from the red light valve unit 317 iss-polarized light with respect to the blue- and red-reflection dichroicmirror coated surface 328, 329. The red light beam enters the side face327 a (a third incidence plane) of the third prism 327, passes throughthe third prism 327, and is reflected totally from the side faceincluding the exit plane 332 to pass through the third prism 327 again.Then, it is reflected from the red-reflection dichroic mirror coatedsurface 329 to pass though the third prism 327 yet again, passes throughthe exit plane 332, and enters the projection lens 306.

The projection lens 306 magnifies and projects the incident light onto ascreen (not shown). Consequently, images of three light beams, each ofwhich is formed by the light valve units 317, 319 and 321, are combinedand displayed as a color image.

According to this embodiment, the color combination optical system 305includes three prisms 325, 326 and 327 that are joined together in theform of a block. This makes it easy to ensure strength and durability,so that the accuracy can be kept high without any deviation after theconvergence has been adjusted. Therefore, images with high quality canbe displayed for a long period of time.

Since the optical paths are filled with glass, the optical path lengthcan be made relatively short (specifically, it can be reduced by twothirds of the optical path length measured when air is used instead ofglass). Also, a relay optical system, which is required for thecross-prism system, is not necessary, thus contributing to a reductionin size of the apparatus.

Moreover, all the reflection planes of the color combination opticalsystem 305 are the side faces of a single prism. Therefore, a favorablefocus can be achieved. In addition, this embodiment can overcome suchproblems of the cross-prism system that a shadow appears due to theinterface between the prisms and color irregularity is caused by thedifference in spectral characteristic between two prism surfaces thatform one reflection plane. Thus, it is possible to provide images withenhanced uniformity. The color combination optical system 305 can beformed basically by joining three prisms having the same shape. Unlikethe cross-prism system, there is no need to align a surface of one prismwith that of the other prism for joining. Accordingly, this embodimenthas advantages over the conventional cross-prism system also due to itslower cost.

In Embodiment 1, the optical path lengths between the projection lens306 and each of the light valve units 317, 319 and 321 are substantiallyequal for the respective light beams. Similarly, the optical pathlengths between each of the light valve units 317, 319 and 321 and thecorresponding light source portions 301, 302 and 303 are substantiallyequal for the respective light beams. Therefore, unlike the cross-prismsystem using a relay optical system, this embodiment does not cause areverse of the light source image of a specific light beam. Thus, it iseasy to achieve high image quality.

The convergence adjustment for combining projection images of therespective light beams is performed generally in the following manner: alight valve unit for one color is fixed, and the remaining light valveunits for the other two colors are adjusted so as to match with theimage formed by the fixed light valve unit. In this embodiment, it ispreferable that the red and green light valve units 317, 321 on bothsides of the blue light valve unit 319 are adjusted, while the bluelight valve unit 319 in the center is fixed. This can facilitateadjustment and minimize the adjustment tolerance of the light valveunits 317, 321.

This embodiment uses a liquid crystal panel having a polarization effectas a light valve. However, note that the present invention is notlimited thereto, and can employ an image display element that displaysimages without relying on polarization. As will be described later, whendichroic mirrors are provided in the color combination optical system,the band of each light beam can be set without causing color mixture ifthose dichroic mirrors transmit p-polarized light for a green light beamand reflect s-polarized light for blue and red light beams, so that itis desirable to use light valves that utilize polarization. In thiscase, a polarization direction converting optical system (see FIG. 6)can be used in the light source portion in Embodiment 1, therebyincreasing the utilization efficiency of light from the light source.The polarization direction converting optical system, which will bedescribed in Embodiment 3, can convert randomly polarized light intopolarized light having a uniform polarization direction.

It is preferable that s-polarized light instead of p-polarized lightshould enter the prisms in the color combination optical system 305 soas to ensure the reflectance of any color light beam with respect to thedichroic mirrors, i.e., a color selection means, in the entire range ofbands. For this reason, in the above example, a blue light beam iss-polarized light with respect to the blue-reflection dichroic mirrorcoated surface (the first dichroic mirror surface) 328, and a red lightbeam also is s-polarized light with respect to the red-reflectiondichroic mirror coated surface (the second dichroic mirror surface) 329.

The color combination optical system of this embodiment is formed sothat a green light beam passes through all the dichroic mirrors. Thespectral characteristic of the blue-reflection dichroic mirror isshifted to a longer wavelength region for s-polarized light than forp-polarized light, and the spectral characteristic of the red-reflectiondichroic mirror is shifted to a shorter wavelength region fors-polarized light than for p-polarized light. Therefore, it isadvantageous that the color combination optical system receives the blueand red light beams as s-polarized light and the green light beam asp-polarized light because a wide range of spectral bands of the dichroicmirrors can be ensured.

In the optical system shown in FIG. 1, the optical path of a red lightbeam can be replaced with that of a blue light beam.

Each of the light source portions 301, 302 and 303 as described aboveprovides a desired color of light by using a filter to select the colorfrom a white light beam emitted from a discharge tube. However, thepresent invention is not limited thereto. For example, no filter isneeded if a discharge tube having spectral distribution suitable foreach color of light is used. Note that in addition to the dischargetube, a laser, an electroluminescence (EL), or the like can be used as alight source.

Embodiment 2

FIG. 3 is a schematic view showing the configuration of aprojection-type image display apparatus according to Embodiment 2 of thepresent invention.

A projection-type image display apparatus 400 of this embodimentincludes a light source portion 401, a color separation optical system402, a light valve portion 403, a color combination optical system 404,and a projection optical system (a projection lens) 405.

The light source portion 401 includes a light source 406 and a reflector407. The light source 406 forms an arc by discharge between electrodesto generate a randomly polarized white light beam. The reflector 407reflects the light beam from the light source 406 in one direction alongits axis of rotational symmetry.

A light beam from the light source portion 401 enters a blue-reflectiondichroic mirror (a first dichroic mirror) 408 of the color separationoptical system 402, where a blue light beam (a third light beam) of theincident white light beam is reflected. Then, the blue light beam isreflected further from a reflection mirror (a third reflection mirror)409 and passes through a condenser lens 410 into a blue light valve unit(a third light valve) 411.

Green and red light beams of the incident white light beam aretransmitted by the blue-reflection dichroic mirror 408 and enter ared-reflection dichroic mirror (a second dichroic mirror) 412, where thered light beam (a second light beam) is reflected. Then, the red lightbeam is reflected further from a reflection mirror (a second reflectionmirror) 413 and passes through a condenser lens 414 into a red lightvalve unit (a second light valve) 415.

The green light beam (a first light beam) is transmitted by thered-reflection dichroic mirror 412, reflected from a reflection mirror(a first reflection mirror) 416, and passes through a condenser lens 417into a green light valve unit (a first light valve) 418.

The light valve portion 403 includes the blue, red and green light valveunits 411, 415 and 418, which are arranged in accordance with therespective light beams. Each of the light valve units 411, 415 and 418includes an entrance polarizing plate 419, a liquid crystal panel 420,and an exit polarizing plate 421, as shown in FIG. 2. The entrancepolarizing plate 419 is rectangular in shape and designed, e.g., totransmit light polarized in the short side direction and to absorb lightpolarized in the direction perpendicular thereto. The light beam passingthrough the entrance polarizing plate 419 enters the liquid crystalpanel 420. The liquid crystal panel 420 has many pixels arranged in theform of an array and can change the polarization direction of thetransmitted light at each pixel aperture with an external signal. Inthis embodiment, when the pixels are not driven, the liquid crystalpanel 420 transmits the incident light while rotating its polarizationdirection by 90 degrees; when the pixels are driven, the liquid crystalpanel 420 transmits the incident light without changing its polarizationdirection. The exit polarizing plate 421 has polarizationcharacteristics in the direction perpendicular to the entrancepolarizing plate 419. In other words, the exit polarizing plate 421 hasa transmission axis in the long side direction of its rectangularoutline and transmits light polarized in this direction. Therefore, thelight beam that has entered the undriven pixel of the liquid crystalpanel 420 and been transmitted with its polarization direction rotatedby 90 degrees can pass through the exit polarizing plate 421 because itis polarized in the direction parallel to the transmission axis. On theother hand, the light that has entered the driven pixel of the liquidcrystal panel 420 and been transmitted with its polarization directionunchanged is absorbed by the exit polarizing plate 421 because it ispolarized in the direction perpendicular to the transmission axis.

The light beams thus transmitted through the light valve portion 403enter the color combination optical system 404.

The color combination optical system 404 is formed by joining threetriangular prisms (i.e., a first prism 422, a second prism 423 and athird prism 424) together. The three prisms are of the same shape, andthe base of each prism is a right triangle having an interior angle of30 degrees (hereinafter, referred to as a vertex angle). As shown inFIG. 3, the three prisms 422, 423 and 424 are joined in this order sothat their vertex angles are next to each other. The side faces 422 a,423 a and 424 a opposite to the vertex angles of the first, second andthird prisms 422, 423 and 424 are opposite to the light valve units 418,415 and 411, respectively. A red-reflection dichroic mirror coatedsurface (a first dichroic mirror surface) 425 is formed at the joiningplane between the first prism 422 and the second prism 423. Similarly, ablue-reflection dichroic mirror coated surface (a second dichroic mirrorsurface) 426 is formed at the joining plane between the second prism 423and the third prism 424. The incidence plane 422 a for the green lightbeam (i.e., the side face of the first prism 422 opposite to the greenlight valve unit 418) is provided with a λ/2 phase-difference plate 428.

The green light beam emitted from the green light valve unit (the firstlight valve) 418 passes through the λ/2 phase-difference plate 428,where its polarization direction is twisted by 90 degrees. The greenlight beam thus twisted is p-polarized light with respect to the red-and blue-reflection dichroic mirror coated surfaces 425, 426. The greenlight beam enters the side face 422 a (a first incidence plane) of thefirst prism 422, passes through the first prism 422, the red-reflectiondichroic mirror coated surface 425, the second prism 423, theblue-reflection dichroic mirror coated surface 426, the third prism 424,and the side face of the third prism (an exit plane 429) in sequence,and enters the projection lens 405, which acts as a projection opticalsystem.

The red light beam emitted from the red light valve unit (the secondlight valve) 415 is s-polarized light with respect to the red- andblue-reflection dichroic mirror coated surfaces 425, 426. The red lightbeam enters the side face 423 a (a second incidence plane) of the secondprism 423, passes through the second prism 423, and is reflected fromthe red-reflection dichroic mirror coated surface 425 to pass throughthe second prism 423 again. Then, it passes through the blue-reflectiondichroic mirror coated surface 426, the third prism 424, and the exitplane 429 and enters the projection lens 405.

The blue light beam emitted from the blue light valve unit (the thirdlight valve) 411 is s-polarized light with respect to the red- andblue-reflection dichroic mirror coated surfaces 425, 426. The blue lightbeam enters the side face 424 a (a third incidence plane) of the thirdprism 424, passes through the third prism 424, and is reflected totallyfrom the side face including the exit plane 429 to pass through thethird prism 424 again. Then, it is reflected from the blue-reflectiondichroic mirror coated surface 426 to pass through the third prism 424yet again, passes through the exit plane 429, and enters the projectionlens 405.

The projection lens 405 magnifies and projects the incident light onto ascreen (not shown). Consequently, images of three light beams, each ofwhich is formed by the light valve units 411, 415 and 418, are combinedand displayed as a color image.

According to this embodiment, the color combination optical system 404includes three prisms 422, 423 and 424 that are joined together in theform of a block. This makes it easy to ensure strength and durability,so that the accuracy can be kept high without any deviation after theconvergence has been adjusted. Therefore, images with high quality canbe displayed for a long period of time.

Since the optical paths are filled with glass, the optical path lengthcan be made relatively short (specifically, it can be reduced by twothirds of the optical path length measured when air is used instead ofglass). Also, a relay optical system, which is required for thecross-prism system, is not necessary, thus contributing to a reductionin size of the apparatus.

Moreover, all the reflection planes of the color combination opticalsystem 404 are the side faces of a single prism. Therefore, a favorablefocus can be achieved. In addition, this embodiment can overcome suchproblems of the cross-prism system that a shadow appears due to theinterface between the prisms and color irregularity is caused by thedifference in spectral characteristic between two prism surfaces thatform one reflection plane. Thus, it is possible to provide images withenhanced uniformity. The color combination optical system 404 can beformed basically by joining three prisms having the same shape. Unlikethe cross-prism system, there is no need to align a surface of one prismwith that of the other prism for joining. Accordingly, this embodimenthas advantages over the conventional cross-prism system also due to itslower cost.

Since the color separation optical system 402 does not include a relayoptical system, the whole size and cost of the apparatus can be reduced.Also, this can prevent color irregularity caused by reversing of thelight source image in the relay optical system.

In Embodiment 2, the optical path lengths between the light sourceportion 401 and each of the light valve units 411, 415 and 418 are equalfor the respective light beams. Similarly, the optical path lengthsbetween the projection lens 405 and each of the light valve units 411,415 and 418 are substantially equal for the respective light beams.

In Embodiment 2, the optical systems are formed so that the optical axisthat goes through the blue-reflection dichroic mirror (the firstdichroic mirror) 408 and the reflection mirror (the first reflectionmirror) 416 is substantially orthogonal to the optical axis that goesthrough the exit plane 429 and the reflection mirror 416. This makes itpossible to reduce the size of the apparatus in the direction parallelto the projection direction. Moreover, a chief ray from the light source406 enters the blue-reflection dichroic mirror 408 at the angle ofincidence smaller than 45 degrees, and thus the optical path lengths ofthe respective light beams are set to be equal in the color separationoptical system 402.

As shown in FIG. 4, the optical systems may be formed so that theoptical axis that goes through the blue-reflection dichroic mirror 408and the reflection mirror (the third reflection mirror) 409 issubstantially parallel to the optical axis that goes through the exitplane 429 and the reflection mirror 416, and a chief ray from the lightsource 406 enters the blue-reflection dichroic mirror 408 at the angleof incidence larger than 45 degrees, like Embodiment 3 to be describedlater, instead of making the optical axis through the blue-reflectiondichroic mirror 408 and the reflection mirror 416 and the optical axisthrough the exit plane 429 and the reflection mirror 416 cross at rightangles. This configuration also allows the optical path lengths betweenthe light source 406 and each of the light valve units 411, 415 and 418to be equal for the respective light beams.

The convergence adjustment for combining projection images of therespective light beams is performed generally in the following manner: alight valve unit for one color is fixed, and the remaining light valveunits for the other two colors are adjusted so as to match with theimage formed by the fixed light valve unit. In this embodiment, it ispreferable that the blue and green light valve units 411, 418 on bothsides of the red light valve unit 415 are adjusted, while the red lightvalve unit 415 in the center is fixed. This can facilitate adjustmentand minimize the adjustment tolerance of the light valve units 411, 418.

This embodiment uses a liquid crystal panel having a polarization effectas a light valve. However, note that the present invention is notlimited thereto, and can employ an image display element that displaysimages without relying on polarization. As will be described later, whendichroic mirrors are provided in the color combination optical system,the band of each light beam can be set without causing color mixture ifthose dichroic mirrors transmit p-polarized light for a green light beamand reflect s-polarized light for blue and red light beams, so that itis desirable to use light valves that utilize polarization. In thiscase, a polarization direction converting optical system (see FIG. 6)can be used in the light source portion in Embodiment 2, therebyincreasing the utilization efficiency of light from the light source.The polarization direction converting optical system, which will bedescribed in Embodiment 3, can convert randomly polarized light intopolarized light having a uniform polarization direction.

It is preferable that, like Embodiment 1, s-polarized light instead ofp-polarized light should enter the prisms in the color combinationoptical system 404 so as to ensure the reflectance of any color lightbeam with respect to the dichroic mirrors, i.e., a color selectionmeans, in the entire range of bands. For this reason, in the aboveexample, a blue light beam is s-polarized light with respect to theblue-reflection dichroic mirror coated surface (the second dichroicmirror surface) 426, and a red light beam also is s-polarized light withrespect to the red-reflection dichroic mirror coated surface (the firstdichroic mirror surface) 425.

The color combination optical system of this embodiment is formed sothat a green light beam passes through all the dichroic mirrors. Thespectral characteristic of the blue-reflection dichroic mirror isshifted to a longer wavelength region for s-polarized light than forp-polarized light, and the spectral characteristic of the red-reflectiondichroic mirror is shifted to a shorter wavelength region fors-polarized light than for p-polarized light. Therefore, it isadvantageous that the color combination optical system receives the blueand red light beams as s-polarized light and the green light beam asp-polarized light because a wide range of spectral bands of the dichroicmirrors can be ensured.

In the optical systems shown in FIGS. 3 and 4, the optical path of a redlight beam can be replaced with that of a blue light beam.

Embodiment 3

FIG. 5 is a schematic view showing the configuration of aprojection-type image display apparatus according to Embodiment 3 of thepresent invention.

A projection-type image display apparatus 500 of this embodimentincludes a light source portion 501, a color separation optical system502, a light valve portion 503, a color combination optical system 504,and a projection optical system (a projection lens) 505.

The light source portion 501 includes a light source 506, a reflector507, an integrator optical system 508, and a polarization directionconverting optical system 509. The light source 506 forms an arc bydischarge between electrodes to generate a randomly polarized whitelight beam. The reflector 507 reflects the light beam from the lightsource 506 in one direction along its axis of rotational symmetry. Theintegrator optical system 508 guides the light beam uniformly from thelight source to light valves. The polarization direction convertingoptical system 509 is provided in the integrator optical system 508 soas to convert the randomly polarized light from the light source intopolarized light having a uniform polarization direction.

Generally, the integrator optical system 508 includes a first lens array510, a second lens array 511 and a condenser lens 512. The first lensarray 510 includes many microlenses arranged closely together on thesame plane, each of which has a shape substantially similar to that ofthe light valve aperture. The second lens array 511 is the same as thefirst lens array 510 in shape. The integrator optical system 508superimposes images of the microlenses on the first lens array 510 ontothe light valve, enabling uniform illumination.

The polarization direction converting optical system 509 is a group ofquadratic prisms arranged in one direction, each of which has aparallelogrammic base, as shown in FIG. 6. A polarizing beam splitterfilm 514 is provided at each of the interfaces (i.e., joining planes)between adjacent prisms that are placed obliquely with respect to theincident light. The polarizing beam splitter film 514 separates theincident light according to the polarization direction. Polarizationdirection converting elements 515 (the λ/2 phase-difference plates maybe used instead) are provided for every other prism on the side of theexit plane. The polarization direction converting element has thefunction of emitting the incident light while rotating its polarizationdirection by 90 degrees. A light beam from the light source passesthrough the prism and enters the polarizing beam splitter film 514,where p-polarized light of the incident light is transmitted ands-polarized light is reflected. The reflected light beam passes throughthe prism into the next polarizing beam splitter film 514, is reflectedtherefrom again, and enters the polarization direction convertingelement 515, which is provided partially on the prism exit plane. Thepolarization direction converting element 515 transmits the incidentlight while rotating its polarization direction by 90 degrees. In thismanner, the polarization direction converting optical system 509converts the incident light into s-polarized light to be emitted.

The polarized light beam thus emitted from the light source portion 501enters a blue-transmission dichroic mirror (a first dichroic mirror) 516of the color separation optical system 502, where a blue light beam (athird light beam) of the incident white light beam is transmitted. Then,the blue light beam is reflected from a reflection mirror (a thirdreflection mirror) 517 and passes through a condenser lens 518 into ablue light valve unit (a third light valve) 519.

Green and red light beams of the incident white light beam are reflectedfrom the blue-transmission dichroic mirror 516 and enter ared-reflection dichroic mirror (a second dichroic mirror) 520, where thered light beam (a second light beam) is reflected. Then, the red lightbeam is reflected further from a reflection mirror (a second reflectionmirror) 521 and passes through a condenser lens 522 into a red lightvalve unit (a second light valve) 523.

The green light beam (a first light beam) is transmitted by thered-reflection dichroic mirror 520, reflected from a reflection mirror(a first reflection mirror) 524, and passes through a condenser lens 525into a green light valve unit (a first light valve) 526.

The light valve portion 503 includes the blue, red and green light valveunits 519, 523 and 526, which are arranged in accordance with therespective light beams. Each of the light valve units 519, 523 and 526includes an entrance polarizing plate 527, a liquid crystal panel 528,and an exit polarizing plate 529, as shown in FIG. 2. The entrancepolarizing plate 527 is rectangular in shape and designed, e.g., totransmit light polarized in the short side direction and to absorb lightpolarized in the direction perpendicular thereto. The light beam passingthrough the entrance polarizing plate 527 enters the liquid crystalpanel 528. The liquid crystal panel 528 has many pixels arranged in theform of an array and can change the polarization direction of thetransmitted light at each pixel aperture with an external signal. Inthis embodiment, when the pixels are not driven, the liquid crystalpanel 528 transmits the incident light while rotating its polarizationdirection by 90 degrees; when the pixels are driven, the liquid crystalpanel 528 transmits the incident light without changing its polarizationdirection. The exit polarizing plate 529 has polarizationcharacteristics in the direction perpendicular to the entrancepolarizing plate 527. In other words, the exit polarizing plate 529 hasa transmission axis in the long side direction of its rectangularoutline and transmits light polarized in this direction. Therefore, thelight beam that has entered the undriven pixel of the liquid crystalpanel 528 and been transmitted with its polarization direction rotatedby 90 degrees can pass through the exit polarizing plate 529 because itis polarized in the direction parallel to the transmission axis. On theother hand, the light beam that has entered the driven pixel of theliquid crystal panel 528 and been transmitted with its polarizationdirection unchanged is absorbed by the exit polarizing plate 529 becauseit is polarized in the direction perpendicular to the transmission axis.

The light beams thus transmitted through the light valve portion 503enter the color combination optical system 504.

The color combination optical system 504 is formed by joining threetriangular prisms (i.e., a first prism 530, a second prism 531 and athird prism 532) together. The three prisms are of the same shape, andthe base of each prism is a right triangle having an interior angle of30 degrees (hereinafter, referred to as a vertex angle). As shown inFIG. 5, the three prisms 530, 531 and 532 are joined in this order sothat their vertex angles are next to each other. The side faces 530 a,531 a and 532 a opposite to the vertex angles of the first, second andthird prisms 530, 531 and 532 are opposite to the light valve units 526,523 and 519, respectively. A red-reflection dichroic mirror coatedsurface (a first dichroic mirror surface) 533 is formed at the joiningplane between the first prism 530 and the second prism 531. Similarly, ablue-reflection dichroic mirror coated surface (a second dichroic mirrorsurface) 534 is formed at the joining plane between the second prism 531and the third prism 532. The incidence plane 530 a for the green lightbeam i.e., the side face of the first prism 530 opposite to the greenlight valve unit 526) is provided with a λ/2 phase-difference plate 536.

The green light beam emitted from the green light valve unit (the firstlight valve) 526 passes through the λ/2 phase-difference plate 536,where its polarization direction is twisted by 90 degrees. The greenlight beam thus twisted is p-polarized light with respect to the red-and blue-reflection dichroic mirror coated surfaces 533, 534. The greenlight beam enters the side face 530 a (a first incidence plane) of thefirst prism 530, passes through the first prism 530, the red-reflectiondichroic mirror coated surface 533, the second prism 531, theblue-reflection dichroic mirror coated surface 534, the third prism 532,and the side face of the third prism (an exit plane 537) in sequence,and enters the projection lens 505, which acts as a projection opticalsystem.

The red light beam emitted from the red light valve unit (the secondlight valve) 523 is s-polarized light with respect to the red- andblue-reflection dichroic mirror coated surfaces 533, 534. The red lightbeam enters the side face 531 a (a second incidence plane) of the secondprism 531, passes through the second prism 531, and is reflected fromthe red-reflection dichroic mirror coated surface 533 to pass throughthe second prism 531 again. Then, it passes through the blue-reflectiondichroic mirror coated surface 534, the third prism 532, and the exitplane 537 and enters the projection lens 505.

The blue light beam emitted from the blue light valve unit (the thirdlight valve) 519 is s-polarized light with respect to the red- andblue-reflection dichroic mirror coated surfaces 533, 534. The blue lightbeam enters the side face 532 a (a third incidence plane) of the thirdprism 532, passes through the third prism 532, and is reflected totallyfrom the side face including the exit plane 537 to pass through thethird prism 532 again. Then, it is reflected from the blue-reflectiondichroic mirror coated surface 534 to pass through the third prism 532yet again, passes through the exit plane 537, and enters the projectionlens 505.

The projection lens 505 magnifies and projects the incident light onto ascreen (not shown). Consequently, images of three light beams, each ofwhich is formed by the light valve units 519, 523 and 526, are combinedand displayed as a color image.

According to this embodiment, the color combination optical system 504includes three prisms 530, 531 and 532 that are joined together in theform of a block. This makes it easy to ensure strength and durability,so that the accuracy can be kept high without any deviation afterconvergence has been adjusted. Therefore, images with high quality canbe displayed for a long period of time.

Since the optical paths are filled with glass, the optical path lengthcan be made relatively short (specifically, it can be reduced by twothirds of the optical path length measured when air is used instead ofglass). Also, a relay optical system, which is required for thecross-prism system, is not necessary, thus contributing to a reductionin size of the apparatus.

Moreover, all the reflection planes of the color combination opticalsystem 504 are the side faces of a single prism. Therefore, a favorablefocus can be achieved. In addition, this embodiment can overcome suchproblems of the cross-prism system that a shadow appears due to theinterface between the prisms and color irregularity is caused by thedifference in spectral characteristic between two prism surfaces thatform one reflection plane. Thus, it is possible to provide images withenhanced uniformity. The color combination optical system 504 can beformed basically by joining three prisms having the same shape. Unlikethe cross-prism system, there is no need to align a surface of one prismwith that of the other prism for joining. Accordingly, this embodimenthas advantages over the conventional cross-prism system also due to itslower cost.

Since the color separation optical system 502 does not include a relayoptical system, the whole size and cost of the apparatus can be reduced.Also, this can prevent color irregularity caused by reversing of thelight source image in the relay optical system.

In Embodiment 3, the optical path lengths between the light sourceportion 501 and each of the light valve units 519, 523 and 526 are equalfor the respective light beams. Similarly, the optical path lengthsbetween the projection lens 505 and each of the light valve units 519,523 and 526 are substantially equal for the respective light beams.

In Embodiment 3, the optical systems are formed so that the optical axisthat goes through the blue-transmission dichroic mirror (the firstdichroic mirror) 516 and the reflection mirror (the third reflectionmirror) 517 is substantially parallel to the optical axis that goesthrough the exit plane 537 and the reflection mirror (the firstreflection mirror) 524. This makes it possible to reduce the size of theapparatus in the direction perpendicular to the projection direction(i.e., the height). Moreover, a chief ray from the light source 506enters the blue-transmission dichroic mirror 516 at the angle ofincidence larger than 45 degrees, and thus the optical path lengths ofthe respective light beams are set to be equal in the color separationoptical system 502.

In Embodiment 3, the integrator optical system 508 and the polarizationdirection converting optical system 509 are mounted in the light sourceportion 501. However, other configurations can be used that function inthe same manner as that described above.

This embodiment uses a liquid crystal panel having a polarization effectas a light valve. However, note that the present invention is notlimited thereto, and can employ an image display element that displaysimages without relying on polarization. As described in Embodiment 2,when dichroic mirrors are provided in the color combination opticalsystem, the band of each light beam can be set without causing colormixture if those dichroic mirrors transmit p-polarized light for a greenlight beam and reflect s-polarized light for blue and red light beams,so that it is desirable to use light valves that utilize polarization.In this case, the polarization direction converting optical system 509that can convert randomly polarized light into polarized light having auniform polarization direction is used in the light source portion,thereby increasing the utilization efficiency of light from the lightsource.

In the optical systems of this embodiment, the optical path of a redlight beam can be replaced with that of a blue light beam.

When giving importance to uniformity of projection images, it isdesirable that the color combination optical system in each ofEmbodiments 1 to 3 is formed as a telecentric optical system.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A projection-type image display apparatuscomprising: a light source portion for emitting a white light beam; acolor separation optical system for separating the white light beam fromthe light source portion into red, green and blue light beams; a lightvalve portion for modulating each of the light beams from the colorseparation optical system; a color combination optical system forcombining the light beams modulated by the light valve portion; and aprojection lens for magnifying and projecting the combined light beam,wherein the color separation optical system comprises at least first andsecond dichroic mirrors and first, second and third reflection mirrors,the light valve portion comprises first, second and third light valves,one each for the respective light beams, the color combination opticalsystem comprises first, second and third triangular prisms, each ofwhich has a vertex angle of about 30 degrees, the first, second andthird prisms are joined together in this order so that side faces ofeach prism that form the vertex angle are brought into contact to makethe vertex angle of one prism next to that of the other prism, each ofjoining planes between the prisms is provided with a dichroic mirrorsurface acting as a color selection means, a side face of each prismopposite to the vertex angle is used as an incidence plane for each ofthe light beams, a side face of the third prism other than the planejoined to the second prism and the incidence plane is used as an exitplane for the combined light beam, the first, second and thirdreflection mirrors are arranged so as to correspond to the first, secondand third light valves, the first, second and third light valves arearranged opposite to the incidence planes of the first, second and thirdprisms, respectively, optical path lengths of the three light beamsbetween the light source portion and the respective light valves aresubstantially equal to one another, optical path lengths of the threelight beams between the incidence planes and the exit plane aresubstantially equal to one another, the first dichroic mirror separatesa third light beam from the white light beam emitted by the light sourceportion, and then the second dichroic mirror separates first and secondlight beams, the first light beam is reflected from the first reflectionmirror, passes through the first light valve, and enters the incidenceplane of the first prism, the second light beam is reflected from thesecond reflection mirror, passes through the second light valve, andenters the incidence plane of the second prism, and the third light beamis reflected from the third reflection mirror, passes through the thirdlight valve, and enters the incidence plane of the third prism.
 2. Theapparatus according to claim 1, wherein a first dichroic mirror surfaceis provided at a joining plane between the first prism and the secondprism, and a second dichroic mirror surface is provided at a joiningplane between the second prism and the third prism, the first light beamentering the incidence plane of the first prism passes through the firstprism, the first dichroic mirror surface, the second prism, the seconddichroic mirror surface, and the third prism in sequence and exits fromthe exit plane, the second light beam entering the incidence plane ofthe second prism passes through the second prism, is reflected from thefirst dichroic mirror surface to pass through the second prism again,passes through the second dichroic mirror surface and the third prism,and exits from the exit plane, and the third light beam entering theincidence plane of the third prism passes through the third prism, isreflected from the side face including the exit plane to pass throughthe third prism again, is reflected from the second dichroic mirrorsurface to pass through the third prism yet again, and exit from theexit plane.
 3. The apparatus according to claim 2, wherein both thelight beams entering the second and third prisms are s-polarized lightwith respect to the first and second dichroic mirror surfaces.
 4. Theapparatus according to claim 2, wherein the light beam entering thefirst prism is p-polarized light with respect to the first and seconddichroic mirror surfaces.
 5. The apparatus according to claim 2, whereinthe light beam entering the first prism is a green light beam.
 6. Theapparatus according to claim 1, wherein the first, second and thirdprisms are of the same shape.
 7. The apparatus according to claim 1,wherein an optical axis that goes through the first dichroic mirror andthe first reflection mirror is substantially orthogonal to an opticalaxis that goes through the first reflection mirror and the exit plane,and a chief ray of the white light beam enters the first dichroic mirrorat an angle of incidence smaller than 45 degrees.
 8. The apparatusaccording to claim 1, wherein an optical axis that goes through thefirst dichroic mirror and the third reflection mirror is substantiallyparallel to an optical axis that goes through the first reflectionmirror and the exit plane, and a chief ray of the white light beamenters the first dichroic mirror at an angle of incidence larger than 45degrees.
 9. The apparatus according to claim 1, wherein light emittedfrom the light source portion is polarized light having a uniformpolarization direction.
 10. The apparatus according to claim 1, whereineach of the first, second and third light valve units comprises at leastan entrance polarizing plate as a polarizer, a transmission-type liquidcrystal panel, and an exit polarizing plate as an analyzer.
 11. Theapparatus according to claim 1, wherein a base of each of the triangularprisms is a right triangle.